Burning flags, burning books (in general).
If your dead mom doesn't matter to me, should I defecate on her grave?
Or, you know, show up at the funeral, pop open the lid, clamber on top of the coffin, drop pants, squat down and do a deuce in the box?
Maybe douse her in gasoline and set it on fire. She was gonna be cremated, anyway, right?
Symbols mean things to people, be that sentimental, religious, nationalistic, etc.
When it comes to holy books, it's sentimental, religious, and (for many) nationalistic, all in one neat little package. And maybe you're pissing off their invisible, undetectable friend, too. Gotta protect their gods' feelings, or they believe their god'll punish them, too. There's always random, miserable daily shit to blame on a god. The ultimate fall-guy.